PROJECT SUMMARY Crohn?s disease (CD) and ulcerative colitis (UC), two major subtypes of inflammatory bowel disease (IBD), are chronic inflammatory disorders of poorly defined etiology. Traditional therapies have focused on amelioration of inflammation, although recent studies have indicated that mucosal healing is an important prognostic endpoint. Moreover, despite aggressive anti-inflammatory treatment, a significant percentage of CD patients develop strictures from intestinal fibrosis and require surgery. Thus, there is an unmet need for alternative treatment options and therapeutic targets. The integrin-binding matricellular protein CCN1 (CYR61) is upregulated in human patients with CD and UC, and is emerging as a key injury response molecule that coordinates multiple aspects of wound healing and tissue repair in colitis. Knockin mice expressing integrin-binding defective CCN1 suffer exacerbated morbidity and mortality in experimental colitis, showing impaired epithelial regeneration, neutrophil persistence, and elevated fibrosis. Mice with Ccn1 deletion in Lgr5+ intestinal stem cells show deficient crypt regeneration following experimental colitis, and mini-gut organoids with this genotype exhibit aberrant Lgr5+ stem cell proliferation. Furthermore, treatment of wild type or Ccn1 mutant mice with purified CCN1 protein significantly accelerates mucosal healing from colitis, suggesting a therapeutic potential of CCN1 for IBD. Based on these findings, we hypothesize that CCN1 plays critical roles in intestinal stem cells and crypt regeneration after injury, and may limit intestinal fibrosis. We will scrutinize this hypothesis in three specific aims: (1) to elucidate the functions of CCN1 in intestinal regeneration and stem cell proliferation and differentiation; (2) to evaluate the role of CCN1 in neutrophil clearance and homeostasis in colitis; and (3) to test the hypothesis that CCN1 can limit and reverse intestinal fibrosis by inducing myofibroblast senescence or apoptosis. These studies will yield new insights into how CCN1 acts on multiple aspects of intestinal injury repair in colitis, and may lead to new treatment strategies and therapeutic targets for IBD.